Time-of-flight mass spectrometers are based on the fundamental principal that ions which have the same initial kinetic energy but different masses will separate when allowed to drift down a field free region, e.g., the length of the flight tube in a conventional time-of-flight mass spectrometer. The ions acquire different velocities according to the mass-to-charge ratio of the ions. Accordingly, lower mass ions will arrive at a detector positioned at the end of the flight tube prior to ions of higher mass. The detector detects the ions collecting the data that yields the mass spectrum for the sample. Traditionally, the detection system is located at the end of the flight tube of a linear time-of-flight mass spectrometer opposite the end of the flight tube where the ions are generated.
Because the ions of different mass-to-charge ratios arrive at the detector at different times continual emission of ions from the ion source into the flight tube is problematic as ions with lower masses may over take slower moving higher mass ions emitted earlier. Accordingly, in the conventional time-of-flight mass spectrometer, it is necessary to allow all ions emitted at a given time to reach the detector before emitting more ions for analysis.
Conventionally the sample that passes into the flight tube is not a continual beam of ions. Usually the ion beam is divided into packets of ions at the ion source. The packets of ions are launched from the ion source at one end of the flight tube into the flight tube using a pulse and wait approach. When using the traditional pulse and wait approach, the release of an ion packet from the source is timed to ensure that the lower mass faster ions of a trailing packet do not pass the higher mass and slower ions of a preceding packet and that the ions of the preceding packet reach the detector before any overlap can occur. Accordingly, the period between release of packets is relatively long as compared to the amount of time for the release. This creates a low duty cycle. As ion sources typically generate ions from a sample continuously in the ion source, only a small portion of the ions generated in the ion source are emitted from the source as ion packets and undergo detection. Thus a significant amount of sample material is wasted and typically sensitivity is reduced. Further in the conventional time-of-flight mass spectrometer the ions of a given packet impinge on the detector in a sequential manner. Recovery of the detector between impacts may require at least a small amount of time. Impact of ions on the detector before recovery leads to degraded isotope resolution.
U.S. Pat. No. 5,396,065 describes a method of addressing the low duty cycle problem by generating an encoded sequence for launching packets of ions before sending them to the field-free region. Upon arrival at the detector, the ion signals are decoded and spectra are reconstructed. This method requires fairly complicated hardware and software algorithms.
U.S. Pat. No. 6,521,887 describes using a position sensitive detector at the end of the flight tube in combination with a system to raster the ion beam to enhance efficiency of detection of ions.
However, the need remains for an improved apparatus and method for time-of-flight mass spectrometry.